Question Re: Hoffman octal bias probes

[mindcrazedbanjo]

I've made 4 of these probes, when they are all in place and I'm checking one valve with my DMM, what do I do with the unconnected leads from the probes on the other valves? do I connect them together to complete the circuit? or is it ok to just let them hang loose unconnected?

[kagliostro]

Don't connect them, only pay attention to the terminals

a little box with a rotary switch to commutate one at a time to the DMM will be of some practical use

Kagliostro

[mindcrazedbanjo]

thank you!

 "only pay attention to the terminals"

do you mean don't let the touch the chasis or something?

[sluckey]

I've made 4 of these probes, when they are all in place and I'm checking one valve with my DMM, what do I do with the unconnected leads from the probes on the other valves?

You must short the two banana plug terminals for each bias checker that doesn't have a DMM connected. Otherwise the cathodes of the unused tubes will be open and the tubes will be switched off. See this pic...



A better solution would be to permanently put a 1 ohm resistor across the terminals of each banana plug (4 resistors total). Now set your DMM to measure millivolts instead of measuring milliamps.

[LooseChange]

Always thought about this... Why not install the one ohm resistor inside the probe. I find on occasion the meter fuse will blow when the amp is switched from Standby.

[sluckey]

Someone sells an octal bias checker with the 1 ohm resistor mounted between the sockets. But it would be a lot of work to dissassemble the sockets to add the resistor. Much easier to put the resistor directly on the banana plug at this point.

[mindcrazedbanjo]

Thx again sluckey for having the patience, that about nails it for me. 

[LooseChange]

I made mine from Dougs design. I'm going to build another with the one ohm resister.

[kagliostro]

do you mean don't let the touch the chasis or something?

yes, I was meaning that

but I remembered badly the schematic, I remembered the resistor was in, not out of the probe

SORRY

Kagliostro